This invention uses a tube as a waveguide for sonic, usually ultrasonic, waves and determines whether the tube has changed its length or been radially deformed along its length by measuring the level and timing of reflected or transmitted waves. In general the device can measure the location along the tube and amount of transverse deflection. As such it can be used either as a displacement measuring device or, by determining when a given level of sound is reached, it can be used as a switch.
Two types of linear or tube like switches are in common use. At many gas stations and along various roads, tubes are placed which indicate when they are run over by a vehicle. These switches use the change in air pressure within the tube to ring a bell or trip a counter. Tape switches use two substantially flat and parallel conductors which, when pushed together, complete the switch. These tape switches arc used on buses and steering wheels, for example, where a momentary depression perpendicular to the switch is to be sensed. The pneumatic tube switches are rate dependent, that is, in many cases they will not function properly if depressed slowly. Tape switches require that the force be approximately perpendicular to the conductors and thus the orientation of the switch is critical. Also tape switches can only be bent about one axis. In neither of the prior art linear switches can the location of the depression be ascertained nor the amount of depression. They arc basically digital devices.
Because of their rate sensitivity, pneumatic tube switches have not found extensive applications beyond those listed above. One use contemplated herein is as an obstruction sensor for use with automobile windows, doors, sunroofs, trunk lids and elevator doors. Pneumatic tube switches are not now used or contemplated for these applications. Tape switches have been considered for some of these applications, however several problems result. The inability to arbitrarily bend the tape switch makes it very difficult to adapt it to irregular contours such as in a trunk lid, for example. Also, it is frequently difficult to place tape switch contacts within weather stripping or other seals which have complex cross section geometries. When tape switches are used it is difficult to maintain the proper orientation of the contacts so that the sensitivity to pressure is constant along the switch. For some cases a wide switch is desirable such as for an obstruction sensor for elevator doors. The use of a tape switch for these cases results in a large switch which is expensive and also difficult to adapt to the door geometry.
Pneumatic tube switches, as well as those of this invention, oilier an advantage over tape switches for applications where electricity can not be tolerated such as in explosive environments. Tape switches, as well as those of this invention, offer advantages over pneumatic tube switches in that both can operate over long distances.
Neither pneumatic tube nor tape switches can measure changes in sensor length which can be accomplished by the sonic tube sensors described below. Applications where this is important include liquid level sensing and weight sensing.
Applications for the displacement sensor described herein include vehicle counting devices for roads; obstruction sensors for windows, doors, sunroofs and trunks; vehicle crash sensors, head contact sensors for headrests; occupant weight measuring sensors for installation in vehicle seats; and fuel level sensors as well as many others. Most if not all of these applications are difficult to solve or unsolvable using conventional technology. The invention described herein solves the above problems by using a tube as a waveguide for sonic waves and measuring the changes to these waves as the tube is stretched or radially compressed. In this manner the problems discussed above, as well as many others, are alleviated or solved by the tubular sonic displacement sensor described in the paragraphs below.